Drunken Monkeys: Some Primates Appear to Prefer Alcohol-Laden Fruits in their Diet

Researchers from the California State University, Northridge (CSUN) and UC Berkeley have determined that at least one specimen of primates, Black-handed spider monkeys, have developed a taste for fruits, in which natural sugars fermented into alcohol, at some point of their evolution.

According to the study, spider monkeys have often preferred fruits that had around 2% alcohol by volume over regular ones. They made this choice without any human interference, the researchers add.

One of the biologists described that they had gathered fruits that the primates ate and collected the samples of their urine to test their theory that some monkeys developed a mechanism to tolerate and metabolise alcohol that is generated in rime fruits naturally under the influence of yeasts consuming sugars. The samples showed secondary metabolites of alcohol - ethyl glucuronide and ethyl sulfate – which means that it did not just go through their digestive system, but was consumed.

"For the first time, we have been able to show, without a shadow of a doubt, that wild primates, with no human interference, consume fruit-containing ethanol", said one of the researchers from CSUN Christina Campbell.

The UC Berkeley biologist, Robert Dudley, noted that fermented fruits likely provide "some physiological benefit" to the spider monkeys since embedded alcohol makes fruits a more high-calorie product. He also suggested that yeasts in these fruits might have an anti-microbial effect and that monkeys benefit from the yeasts predigesting the fruit.

At the same time, the biologists are currently unaware if the alcohol in fruits affects the primates the way it affects humans, but doubt that they are getting wasted eating them.

"What we don't know is how much of it they're eating and what the effects are behaviorally and physiologically," Campbell said. "They're probably not getting drunk, because their guts are filling before they reach inebriating levels," a biologist from UC Berkeley, Robert Dudley, said.

The new findings back a 2014 theory by Robert Dudley called "drunken monkey," according to which humans might have a taste for alcoholic beverages due to their primate ancestors consuming them to benefit from receiving more calories. While the theory is not yet 100% proven, Dudley previously suggested that consumption of fermented fruits drove the evolution of our primate ancestors as they could gather fallen fruits that started to ferment alcohol inside them.

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